Digital imagery processing systems, such as those employed for processing digitize color photographic images, customarily digitized images by way of an opto-electronic scanner, the output of which is encoded to some prescribed digital encoding resolution (or digital code width) that encompasses a range of values over which the contents of a scene, such as that captured on a (color) photographic recording medium, may vary. As diagrammatically illustrated in FIG. 1, for a typical color photographic negative film, this range of values R is less than the density vs. exposure latitude of the film, but is sufficiently wide to encompass those film density values that can be expected to be encountered for a typical scene. Then, by means of a preliminary image operator, such as a scene balancing mechanism, the digitized image is mapped into a set of digital codes, each of which has a digital resolution corresponding to the dynamic range of a digitized image data base (e.g. frame store), the contents of which may be adjusted in the course of driving an output device, for example enabling a print engine to output a high quality color print.
As an example, as further illustrated in FIG. 1, the mapping of the quantized output of a digital image scanning device may translate the contents of a given portion of the density vs. log exposure characteristic of a color photographic negative film into a database digital resolution of eights bits per color per pixel (twenty-four bits per pixel), with a value of 255 corresponding to maximum 100% white reflectance (normally defined as a perfect (100%) non-fluorescent white reflecting diffuser). Other density values representing lesser reflectances are encoded relative to this maximum down to a code value of zero, corresponding to a low reflectance value (e.g. black).
As a consequence, if, in addition to basic content of the scene, an image contains specular highlights (e.g. a reflection from a car bumper, identified at exposure line SH in FIG. 1), their associated pixel values will be maximally encoded or `clipped` at 255--the same as that for the above-referenced 100% white reflectance, so that a portion of their reflectance characteristics is lost. In addition, supplemental scene balance image processing, as may be necessary to accommodate the parameters of a particular output device, may operate so to as adjust one or more pixel values upwardly, causing a further increase in the number of pixel values whose encoded values are maximal. Unfortunately, once a data value has been maximized it cannot be shifted to a lower value without similarly affecting other like valued data, so that the content of an image reproduced (printed or displayed) from the digitized image is degraded.
As a further consequence, if, in addition to basic content of the scene, an image contains unusually low reflectances or areas of objects in shadow light (e.g. shadow object identified at exposure line SS in FIG. 1), their associated pixel values will be minimally encoded or `clipped` at 0, so that a portion of their reflectance characteristics is lost. In addition, supplemental scene balance image processing, as may be necessary to accommodate the parameters of a particular output device, may operate so to as adjust one or more pixel values downwardly, causing a further increase in the number of pixel values whose encoded values are minimal. Unfortunately, once a data value has been minimized it cannot be shifted to a higher value without similarly affecting other like valued data, so that the content of an image reproduced (printed or displayed) from the digitized image is degraded.